Useful to move many files (thousands or millions files) over ssh. Faster than scp because this way you save a lot of tcp connection establishments (syn/ack packets).
If using a fast lan (I have just tested gigabyte ethernet) it is faster to not compress the data so the command would be:
tar -cf - /home/user/test | ssh user@sshServer 'cd /tmp; tar xf -'

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